Photoluminescence imaging or photoluminescence spectroscopy is a contactless, nondestructive method of probing the electronic structure of materials, such as silicon semiconductor wafers, as well as other workpieces and materials. In a typical photoluminescence process, light is directed onto a wafer or other workpiece or sample (hereinafter collectively referred to as a “wafer”), where at least some of the light is absorbed. The absorbed light imparts excess energy into the material via a process of “photo-excitation.” This excess energy is dissipated by the wafer through a series of pathways; one such pathway is the emission of light, or photoluminescence. The intensity and spectral content of this photoluminescence is directly related to various material properties of the wafer.
Photoluminescence imaging processes may be used to identify and quantify defects and contaminants present in the wafer based on spatial variations in the photoluminescence images produced. One photoluminescence imaging process, as described in International Application Number PCT/GB97/02388 (publication number WO 98/11425), which is incorporated herein by reference, involves probing the surface and/or the sub-surface bulk region of the wafer with one or more lasers of varying excitation wavelengths. A laser of a given wavelength is directed into the wafer and penetrates the wafer to a given depth. Return light emitted from excited regions of the wafer is detected and quantified by a detection system. Images of the measured return light, including spatial images of defects and contaminants in the wafer, may then be produced by the detection system or by an associated image-producing system.
While these photoluminescent images may effectively identify defects and contaminants in the wafer as a whole, it is sometimes difficult to readily identify the location and concentration of defects in a specific material layer of the wafer. For example, if two or more material layers of the wafer are penetrated by a laser, images of defects in the second material layer may obscure images of defects in the first material layer. This can be problematic if detailed defect data about only the first material layer is desired. Thus, there is a need to be able to obtain more accurate measurements of the location and concentration of defects in a specific material layer or layers of a wafer.